Circuits used in many electronic devices, for example, cellular phones and radios, produce, receive, or function with high frequency signals as well as low frequency signals. Integration of high and low frequency circuits typically involve the use of hybrid substrates, with low frequency devices formed on FR4, for example, and high frequency devices formed on RT/Duroid©, for example. Both the low and high frequency signals may be transmitted across a substrate or printed circuit board by metal traces; however, while low frequency signals may be transmitted along a single metal trace, the high frequency signal is typically transmitted by multiple metal traces which form a waveguide structure, such as a microstrip or coplanar trace. The coplanar trace is one in which two or more metal traces are formed on the same surface, thereby guiding an electromagnetic signal between them. These metal traces typically transmit the high frequency signal between circuits such as amplifiers, oscillators, and mixers positioned on a printed circuit board.
Coplanar circuit structures conventionally include coplanar waveguide structures and slotline structures. A coplanar waveguide structure has one or more spaced longitudinal coplanar strip signal conductors positioned between and separated from two longitudinal coplanar ground conductors by respective gap widths, wherein the ground conductors are typically much wider than the gaps. A slotline structure has two spaced longitudinal coplanar conductors having a gap therebetween, wherein the gap is typically much smaller than the lateral width of the conductors.
The metal traces of a coplanar strip transmission line conventionally are formed on a dielectric material, such as a printed circuit board. The high frequency signal exists as an electromagnetic field in the gap between the metal traces. The gap includes the dielectric material as well as air between and above the metal traces. The existence of the electric field in the dielectric material results in undesirable losses in signal strength. This is exacerbated by the electric field naturally concentrating in the higher dielectric constant material over the lower dielectric air.
This loss in signal strength may be reduced by forming the circuitry (both low and high frequency) on a high frequency substrate. However, substrates typically used for high frequency signals are much more costly than substrates typically used for low frequency signals. For circuit board applications, the loss is reduced by using high frequency substrates such as RT/duroid® from Rogers Corp, instead of traditional circuit board material, such as FR4.
Another known approach to reduce this loss in signal strength is to form a high frequency substrate material, e.g., RT/duroid®, on or over a low frequency substrate, e.g., an FR4 material. High frequency circuitry would be formed on the high frequency substrate material and the low frequency circuitry would be formed on the low frequency substrate. However, while this may be less costly than using an entire substrate of high frequency substrate material, it is still a complicated and costly process.
Accordingly, it is desirable to provide a coplanar strip transmission line for transmission of high frequency signals on substrates typically used for lower frequency devices. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.